I just did the upgrade to the new 6.5 version and now have problem connecting my viewer to the server, when i click sig-in and enter the credentials it show that it's connection but after a few seconds i'm back at Sign-in. This is working on only one computer and on that computer i can access my shared address book.
P.S. For anyone else to note, if you used address book sync in the past (i.e. in version 6.3.x.x and server 2.2 and earlier) and now have upgraded to the new version you must upgrade both Viewer and Server and then follow this address book sync upgrade guide. In a nutshell, you just need to export your existing address book, then set up address book sync anew - in the new version it is set up differently than before - and import your address book back into the system.